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Solubilization of excess activated sludge by self-digestion
PII: S0043-1354(98)00408-4
Wat. Res. Vol. 33, No. 8, pp. 1864±1870, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
SOLUBILIZATION OF EXCESS ACTIVATED SLUDGE BY SELF-DIGESTION YUKO SAIKI1*, SEIJI IMABAYASHI1, CHIKAKO IWABUCHI1, YASUSHI KITAGAWA1, YASUSHI OKUMURA2 and HIKARU KAWAMURA1 1 Process Engineering Research and Development Laboratory, Asahi Breweries Ltd., 1-1-21 Midori, Moriya-machi, Kitasoma-gun, Ibaraki 302-0106, Japan and 2Bioscience Research and Development Laboratory, Asahi Breweries Ltd., 1-1-21 Midori, Moriya-machi, Kitasoma-gun, Ibaraki 302-0106, Japan
(First received May 1998; accepted in revised form September 1998) AbstractÐA new approach to the solubilization of excess activated sludge by self-digestion was studied to reduce the amount of sludge produced in the activated-sludge-treatment process. Excess sludge was eectively made soluble by anaerobic incubation at 608C after the addition of NaOH at a ®nal concentration of 0.01 N. Under these conditions, approximately 40% of the sludge became soluble within 4 days. During self-digestion, solids in the sludge were thickened by ¯otation, and a clear liquor was obtained. This process was evaluated with a bench scale apparatus. The system consisted of an up¯ow column as the solubilizer. The resulting solubilization rate was approximately 20% at the hydraulic retention time of 24 h. This was comparable to the value obtained in batch experiments under identical conditions. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐactivated sludge, solubilization, self-digestion, sludge reduction, up¯ow anaerobic sludge blanket reactor (UASB), ¯otation, solid/liquid separation, methane fermentation
INTRODUCTION
A major problem in today's wastewater treatment plants is the management of the excess activated sludge produced by aerobic treatment. Although solubilization of excess activated sludge by thermal treatment (Haug, 1978; Hiraoka et al., 1985; Li and Noike, 1989) has been an actively promoted process in which sludge is solubilized thermally then digested anaerobically, it requires a large quantity of energy. Recent research on solubilization has shown that chemical (Nagai and Nishio, 1985; Rajan et al., 1989; Ray et al., 1990), mechanical (Shimizu et al., 1992a,b,c) or microbiological (Kitazume et al., 1991) solubilization of sludge prior to anaerobic treatment improves digestive eciency to some extent. Ray et al. (1990) showed that a low-level, ambient temperature alkaline solubilization process improved volatile solids (VS) removal and gas production when compared to no pretreatment of the sludge. Their ®ndings also indicate that the anaerobic digestion time could be shortened to 7.5 days. Kitazume et al. (1991) isolated acid-forming anaerobes which made more than 50% of the volatile solids in activated sludge *Author to whom all correspondence should be addressed. [Fax: +81-297-461524].
soluble. Inoculation of these anaerobes accelerated the initial digestion rate by more than 20% in batch experiments lasting 30 days. Such technologies, however, are not frequently used in newly built sewage treatment plants because they are not self-sustaining in terms of energy balance, and they require a large site space. Also, these procedures do not solve the problem of subsequent liquid/solid separation. If an economical solubilization process for excess sludge that has improved solid/liquid separation could be established, the resulting solubilization liquor would be treatable by the use of an up¯ow anaerobic sludge blanket (UASB) reactor, solubilization becoming more feasible. We focused on self-digestion of bacteria that are main component of activated sludge. Self-digestion generally occurs when the growth environment such as temperature, pH, the oxigenic condition, is changed. The advantages it has over other methods would be the low running cost, methane fermentability of solubilized compounds and the possibility of application of gaseous products to an eective solid/liquid separation. In this study, the optimum self-digestion conditions of excess activated sludge were investigated using the batch treatment method, then continuous sludge solubilization was examined using a bench
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Solubilization of excess activated sludge Table 1. Components of the activated excess sludge used in the batch experiments Soluble TOC (mg/l) Total TOC (mg/l) VFA (mg/l) Sludge concentration (%) Ash (mg/l) pH
62.1 1950 N.D.$ 0.79 4003 8.0
$
Not detected.
scale apparatus in order to evaluate the practical value of self-digestion. A new sludge reducing system which combines self-digestion and UASB treatment is proposed. EXPERIMENTAL
Batch treatment method The excess activated sludge used in the experiment was drawn from our brewery wastewater treatment plant and stored at 48C until use. The components of the sludge are shown in Table 1. The solubilization procedure in the batch experiment was as follows: 100 ml of the excess activated sludge was placed in a medium bottle (100 ml) and a small amount of NaOH or HCl (maximum 0.03 N) was added. The bottle was incubated at a temperature of 50 to 808C anaerobically using AnaeroPack (Mitsubishi Gas Chemical Co.). After incubation, the mixed liquor was centrifuged at 2,000 rpm for 15 min then ®ltered through a membrane ®lter (Toyo Roshi Kaisya, pore size 0.45 mm) to obtain the solubilization liquor for the determination of the total organic carbon (TOC) and the volatile fatty acid (VFA) concentrations. TOC concentration was determined by a TOC analyzer (Shimadzu, TOC-5000) equipped with a solid sample module (Shimadzu, SSM-5000A) and the VFA concentration by an organic acid analyzing system (Shimadzu, LC-10). The solubilization ratio was de®ned as the TOC concentration of the ®ltrate/the TOC concentration of the sludge before treatment 100. Similarly, the VFA formation ratio was de®ned as the TOC concentration of the VFA in the ®ltrate/the TOC concentration of the activated sludge 100.
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Methane fermentability measurement The granular sludge and UASB euent used for methane fermentation were obtained from the UASB reactor of the brewery's wastewater plant. The procedure used to measure methane fermentability was as follows: the sludge solubilized for 2 days was centrifuged at 2000 rpm for 15 min, after which the supernatant was diluted with the UASB euent to a COD value of 0.5 g/l. One l of this liquor then was added to 4 g of the granular sludge and the whole stirred at 358C under anaerobic conditions. The gas generated was bubbled through a 0.2 N solution of sodium hydroxide to eliminate carbon dioxide, after which the volume of methane gas was measured. Continuous treatment apparatus The experimental apparatus consisted of a semicontinuous, up¯ow column as the solubilizer, details of which are shown schematically in Fig. 1. It had a diameter of 257 mm, a liquid volume of approximately 50 l and was maintained at 60±638C with belt-heaters to promote selfdigestion. After the addition of sodium hydroxide at the ®nal concentration of 0.01 N and heating to 63±708C, sludge was supplied from the bottom of the column. A tube of the same diameter was attached at the top of the column, through which the solubilization liquor and the residual sludge was removed by over¯ow to a collection basin. This system was operated on an automatic 2 h cycle. For the ®rst 20 min of each cycle, the pump fed sludge into the column then stopped. After 100 min, it fed sludge again. The ¯ow rate was set at 200 ml/h, the hydraulic retention time being maintained for 24 h. The sludge used in this experiment was drawn from the same wastewater plant as the batch experiments and supplied for the apparatus directly.
RESULTS AND DISCUSSION
Conditions for self-digestion Figure 2 shows the time course of the solubilization ratio of excess sludge under several conditions. Solubilization proceeded rapidly for one day, after which the rate decreased. The solubilization ratio reached the maximum value in 4 days.
Fig. 1. Continuous treatment apparatus.
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Fig. 2. Time course of solubilization. Solubilized w: at 808C and 0.03 N NaOH; .: at 608C and 0.01 N NaOH; Q: at 608C without NaOH; R: at 378C without NaOH.
The eects of acid/alkali and the temperatures on solubilization were also examined. The results after 2 days of incubation are shown in Fig. 3. The solubility of the sludge increased with the rise in temperature and pH, whereas the VFA formation ratio was fairly high under neutral conditions. These ®ndings suggest that there were two types of solubilization; self-digestion accompanied by formation of these VFAs and alkaline solubilization which proceeds at high temperatures with poor VFA formation. When the liquor is to be treated anaerobically, the self-digestion method is
preferable to alkaline solubilization because the VFAs generated during self-digestion are the good substrates for methane fermentation. The less cost for solubilization is also favorable. In addition, a nearly colorless, clear solubilization liquor may be separated out by ¯otation without a conventional solid/liquid separation process. As far as the sludge was solubilized by selfdigestion, it seemed that the reactions proceeded insuciently at 508C, particularly without NaOH or HCl. A certain level of shock caused by changes of the surroundings might be required for selfdigestion. Above 708C with NaOH, the ratio of non-VFA was increased, indicating that the COD removal would be low in the anaerobic treatment. Thus, several conditions, such as a temperature of 608C and 0.01 N NaOH, temperatures of 70 or 808C without adding sodium hydroxide, were considered to be suitable for actual solubilization, evaluated on the basis of the solubilization ratio and VFA formation. The latter was advantageous in the point that the apparatus for alkaline addition is not required. The former, however, showed more stable results in spite of daily ¯uctuations of the sludge characteristics. Moreover, the temperature can be maintained easier in the case that the solubilization is performed with an open column. Therefore, we used the following conditions for the subsequent experiments: a temperature of 608C, 0.01 N NaOH (®nal concentration) and a retention time of 1 to 2 days.
Fig. 3. Eects of acid/alkali dose and temperature on solubilization. The additions and their ®nal concentrations were; (A) HCl 0.03 N; (B) HCl 0.02 N; (C) HCl 0.01 N; (D) none; (E) NaOH 0.01 N; (F) NaOH 0.02 N and (G) NaOH 0.03 N.
Solubilization of excess activated sludge
Fig. 4. Methane yield from the solubilization liquor. Solubilized .: at 608C with 0.01 N NaOH for 2 days; *: at 608C without NaOH for 2 days; R: at 808C with 0.03 N NaOH for 2 days; w: control with sodium acetate as the substrate; q: blank.
Methane fermentability Figure 4 shows the methane production from the solubilized matter. The rates were signi®cantly aected by the solubilization conditions. At 608C and 0.01 N NaOH, the rate was almost the same as in the control experiment with authentic acetate in place of the solubilization liquor. Judging from this, the methane fermentability of the liquor under this condition was excellent. At 608C without NaOH, the methane fermentation rate was 77% at 5 h as compared to authentic acetate, though there was no signi®cant dierence in the proportion of VFAs in the substrates. As the explanation for this reduction in the rate, the degradation of biopolymeric substances to lower-molecular-weight-compound might have been proceeded insuciently, or less likely, inhibitory substances to methane fermentation had been formed in the series of solubilization reactions. To clarify this, further investigations would be needed. On the other hand, when the sludge was solubilized at 808C and 0.03 N NaOH, methane production markedly decreased to 35% at 5 h as compared to authentic acetate. It was con®rmed that drastic heating and excess addition of NaOH caused the formation of substances that was not readily convertible to methane. These results indicate that the conditions of 608C and 0.01 N NaOH are suitable for the ecient anaerobic treatment followed by the solubilization.
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bilization by self-digestion, there is a possibility that no special gas generation process is required because enough gas was formed and the ¯otation occurred during self-digestion. If the ¯oating residue can be removed eectively in larger plants, a solid/liquid separation process would be unnecessary. The suspended solids (SS) value of the solubilization liquor was less than 15 mg/l in the batch experiments. Flotation of the sludge occurred within 1 h of heating. When ¯otation proceeded suciently, the speci®c gravity of the ¯oating sludge was reduced to 0.6±0.7 due to the gas bubbles existed in the ¯oating sludge. The thickening eciency, or the solid content of the ¯oating sludge paralleled the increasing depth of the sludge suspension in the containers (Fig. 5). The concentration and the volume of the sludge had no eect on the ®nal solid content of the ¯oating sludge cakes. Above a solid content of 10%, the sludge residue masses were sti and the solubilization liquor could be separated easily. Preliminary results indicate that the thickened residual sludge produced by this process had improved dewaterability than non-treated sludge. The ¯otation of solids was not always followed by self-digestion. The rise of the temperature, the lowering of the pH of the sludge suspension or both caused immediate ¯otation of the solids. This suggests that the release of carbon dioxide dissolved in the sludge liquor caused the ¯otation. Nevertheless, ¯otation occurred along with selfdigestion when the original carbon dioxide was removed by replacing the sludge liquor with distilled water. In this self-digestion process, it can be consequently said that both the original carbon dioxide and the gaseous product of self-digestion contributed to the ¯otation thickening and made the ¯oating sludge stable and sti.
Liquid/solid separation In a liquid/solid separation process prior to dewatering, ¯otation is used for the thickening of the sludge occasionally. The formation of gas bubbles which lift up the solids is said to be one of the critical steps for the eective ¯otation thickening. In the conventional ¯otation thickening techniques, physical, chemical and electrolytic methods are used for the gas generation. In case of solu-
Fig. 5. Correlation of initial depth of the sludge and solid content of the ¯oating sludge. The initial sludge concentrations and the volumes were Q: 0.71%, 100 ml; q: 0.90%, 100 ml; w: 0.90%, 500 ml.
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Fig. 6. Solids behavior in the solubilizer. (a) Solids fed into the solubilization column in each cycle were initially settled and thickened in the solubilizer. (b) As self-digestion proceeds, gas bubbles gradually form in the solids layers and lift up the solids. (c) The ¯otated solids undergo further solubilization then over¯ow from the top of the solubilizer.
Continuous treatment The behavior of the solids in the solubilization column was shown in Fig. 6. The proportion of the submerged sludge to the total ¯oating sludge was around 70%, indicating that the average speci®c gravity of the ¯oating sludge was around 0.7. When the ¯oating sludge was accumulated in the column, it was pushed and collapsed by the wall of the exhaustion tube and then ®nally exhausted from the top of the column along with the liquor. In the collection basin, the residue was immediately settled at the bottom of it because the gas bubbles adhering to the solids were released. In this experiments, the solubilization liquor could be separated from the solid±liquid mixture in the basin only by pumping. Operational results are shown in Fig. 7. The sludge concentration was varied between 0.7 and 1.4%, as it was drawn and supplied for the apparatus directly. Solubilization of approximately 20%
and VFA formation ratios of about 10% were obtained at a hydraulic retention time of 24 h. These ®ndings are very similar to those obtained in batch experiments under identical conditions. The solid content of the residue (the solids after selfdigestion) ranged from 15 to 17% at the top of the column. The SS values of the solubilization liquor ranged from 25 to 125 mg/l. On the basis of the fact that excess activated sludge is composed mainly of bacteria, the sludge could be solubilized by self-digestion. There are several merits to using self-digestion. One is that the conditions for self-digestion are milder than in other solubilization methods because self-digestion may occur when there is a change in the conditions as in pH and temperatures. When self-digestion occurred, the biopolymeric substances were hydrolyzed, forming VFAs that were easily converted to methane under anaerobic conditions. The usefulness of ¯otation of the solids during self-digestion also should be noted because it seems to be eective
Fig. 7. Daily solubilization and VFA formation ratios. w: solubilization ratio; .: VFA formation ratio. The solid and broken bars on the right show the range of the data obtained from the batch experiments under identical conditions.
Solubilization of excess activated sludge
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Fig. 8. Proposed wastewater treatment system.
for solid/liquid separation. Whereas most other research has regarded solubilization as pretreatment for anaerobic digestion, this approach may enable the combination of sludge solubilization and UASB treatment. A ¯ow diagram of the proposed process is shown in Fig. 8. This system combines sludge solubilization with the present wastewater treatment process that consists of aerobic treatment followed by anaerobic treatment using UASB. The excess activated sludge drawn from the sedimentation tank is solubilized and reduced by self-digestion. After solid/liquid separation, methane energy is recovered from the solubilized matter using a UASB reactor. With this system, excess sludge can be reduced economically, especially in the wastewater treatment plants that have anaerobic treatment systems. The time required for its treatment, including secondary UASB treatment, is markedly reduced; 2 to 5 days, depending on the solubilization ratio desired. Moreover, the low operation cost, ability to recover methane and the simplicity of the bench scale unit make it suitable for treating large quantities of sludge by self-digestion. Several factors, however, require clari®cation. In this study, the residual solids exhausted from the continuous apparatus were separated from the solubilization liquor by settling. If the ¯oating cake can be removed directly from the top of the solubilization column, the proposed process might be simpli®ed even further. Also, extensive dewaterability analyses have yet to be performed, the results of which may signi®cantly aect the cost of sludge disposal. Moreover, the odor problem should be considered when putting this system to practical use especially in the town area. The eectiveness of this process on sludges from other treatment plants also must be investigated. A pilot scale apparatus of the proposed self-digestion system has been constructed and is currently being tested. It is hoped that an ongoing approach will produce favorable ®ndings.
SUMMARY AND CONCLUSIONS
Our study of a self-digestion process to reduce excess sludge produced the following conclusions: 1. Solubilization (20±40%) of excess sludge by selfdigestion was possible. The selected self-digestion conditions were; a temperature of 608C, an anaerobic incubation time of 4 days and an NaOH concentration of 0.01 N. These conditions ensured approximately 40% solubilization of the organic matter in the excess sludge. 2. The gas bubbles formed during self-digestion caused thickening of the residual solids by ¯otation. 3. The subnatant liquor was eectively treated with anaerobic granular sludge, forming methane. 4. In the continuous treatment with a retention time of 24 h, the solubilization ratio was almost the same as that for the batch experiments done under identical conditions. AcknowledgementsÐThis research, one subject in the National Project for Development of Biosensors in the Food Industry, was supported in part by a grant from the Japanese Ministry of Agriculture, Forestry and Fisheries to Asahi Breweries, through the project implementation body, the Society for Techno-Innovation of Agriculture, Forestry and Fisheries.
REFERENCES
Haug R. T. (1978) Eect of thermal pretreatments on digestibility and dewaterability of organic sludge. JWPCF 50, 73±85. Hiraoka M., Takeda N., Sakai S. and Yasuda A. (1985) Highly ecient anaerobic digestion with thermal pretreatment. Water Sci. Technol. 17, 529±539. Kitazume M., Ueyama S. and Benno Y. (1991) Solubilization of activated sludge by isolated acid-forming anaerobes. Hakkokogaku 69, 363±372 (in Japanese with English abstract). Li Y. and Noike T. (1989) The eect of thermal pretreatment and retention time on the degradation of waste activated sludge in anaerobic digestion. J. Water Poll. Res. 12, 112±121 (in Japanese with English abstract).
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Nagai S. and Nishio S. (1985) Rapid methane fermentation. J. Water Waste 27, 1134±1143 (in Japanese). Rajan R. V., Lin J.-G. and Ray B. T. (1989) Low-level chemical pretreatment for enhanced sludge solubilization. J. Water Pollut. Control Fed. 61, 1678. Ray B. T., Lin J. G. and Rajan R. V. (1990) Low-level alkaline solubilization for enhanced anaerobic digestion. JWPCF 62, 81±87. Shimizu T., Kudo K. and Nasu Y. (1992a) Ultrasonic pre-
treatment of waste activated sludge for anaerobic digestion. J. Water Waste 34, 221±226 (in Japanese). Shimizu T., Kudo K. and Nasu Y. (1992b) Anaerobic digestion of a solubilized waste activated sludge. J. Water Waste 34, 413±418 (in Japanese). Shimizu T., Kudo K. and Nasu Y. (1992c) Improvement of eciency for anaerobic digestion process of waste activated sludge. J. Water Waste 34, 497±502 (in Japanese).
Wat. Res. Vol. 33, No. 8, pp. 1864±1870, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
SOLUBILIZATION OF EXCESS ACTIVATED SLUDGE BY SELF-DIGESTION YUKO SAIKI1*, SEIJI IMABAYASHI1, CHIKAKO IWABUCHI1, YASUSHI KITAGAWA1, YASUSHI OKUMURA2 and HIKARU KAWAMURA1 1 Process Engineering Research and Development Laboratory, Asahi Breweries Ltd., 1-1-21 Midori, Moriya-machi, Kitasoma-gun, Ibaraki 302-0106, Japan and 2Bioscience Research and Development Laboratory, Asahi Breweries Ltd., 1-1-21 Midori, Moriya-machi, Kitasoma-gun, Ibaraki 302-0106, Japan
(First received May 1998; accepted in revised form September 1998) AbstractÐA new approach to the solubilization of excess activated sludge by self-digestion was studied to reduce the amount of sludge produced in the activated-sludge-treatment process. Excess sludge was eectively made soluble by anaerobic incubation at 608C after the addition of NaOH at a ®nal concentration of 0.01 N. Under these conditions, approximately 40% of the sludge became soluble within 4 days. During self-digestion, solids in the sludge were thickened by ¯otation, and a clear liquor was obtained. This process was evaluated with a bench scale apparatus. The system consisted of an up¯ow column as the solubilizer. The resulting solubilization rate was approximately 20% at the hydraulic retention time of 24 h. This was comparable to the value obtained in batch experiments under identical conditions. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐactivated sludge, solubilization, self-digestion, sludge reduction, up¯ow anaerobic sludge blanket reactor (UASB), ¯otation, solid/liquid separation, methane fermentation
INTRODUCTION
A major problem in today's wastewater treatment plants is the management of the excess activated sludge produced by aerobic treatment. Although solubilization of excess activated sludge by thermal treatment (Haug, 1978; Hiraoka et al., 1985; Li and Noike, 1989) has been an actively promoted process in which sludge is solubilized thermally then digested anaerobically, it requires a large quantity of energy. Recent research on solubilization has shown that chemical (Nagai and Nishio, 1985; Rajan et al., 1989; Ray et al., 1990), mechanical (Shimizu et al., 1992a,b,c) or microbiological (Kitazume et al., 1991) solubilization of sludge prior to anaerobic treatment improves digestive eciency to some extent. Ray et al. (1990) showed that a low-level, ambient temperature alkaline solubilization process improved volatile solids (VS) removal and gas production when compared to no pretreatment of the sludge. Their ®ndings also indicate that the anaerobic digestion time could be shortened to 7.5 days. Kitazume et al. (1991) isolated acid-forming anaerobes which made more than 50% of the volatile solids in activated sludge *Author to whom all correspondence should be addressed. [Fax: +81-297-461524].
soluble. Inoculation of these anaerobes accelerated the initial digestion rate by more than 20% in batch experiments lasting 30 days. Such technologies, however, are not frequently used in newly built sewage treatment plants because they are not self-sustaining in terms of energy balance, and they require a large site space. Also, these procedures do not solve the problem of subsequent liquid/solid separation. If an economical solubilization process for excess sludge that has improved solid/liquid separation could be established, the resulting solubilization liquor would be treatable by the use of an up¯ow anaerobic sludge blanket (UASB) reactor, solubilization becoming more feasible. We focused on self-digestion of bacteria that are main component of activated sludge. Self-digestion generally occurs when the growth environment such as temperature, pH, the oxigenic condition, is changed. The advantages it has over other methods would be the low running cost, methane fermentability of solubilized compounds and the possibility of application of gaseous products to an eective solid/liquid separation. In this study, the optimum self-digestion conditions of excess activated sludge were investigated using the batch treatment method, then continuous sludge solubilization was examined using a bench
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Solubilization of excess activated sludge Table 1. Components of the activated excess sludge used in the batch experiments Soluble TOC (mg/l) Total TOC (mg/l) VFA (mg/l) Sludge concentration (%) Ash (mg/l) pH
62.1 1950 N.D.$ 0.79 4003 8.0
$
Not detected.
scale apparatus in order to evaluate the practical value of self-digestion. A new sludge reducing system which combines self-digestion and UASB treatment is proposed. EXPERIMENTAL
Batch treatment method The excess activated sludge used in the experiment was drawn from our brewery wastewater treatment plant and stored at 48C until use. The components of the sludge are shown in Table 1. The solubilization procedure in the batch experiment was as follows: 100 ml of the excess activated sludge was placed in a medium bottle (100 ml) and a small amount of NaOH or HCl (maximum 0.03 N) was added. The bottle was incubated at a temperature of 50 to 808C anaerobically using AnaeroPack (Mitsubishi Gas Chemical Co.). After incubation, the mixed liquor was centrifuged at 2,000 rpm for 15 min then ®ltered through a membrane ®lter (Toyo Roshi Kaisya, pore size 0.45 mm) to obtain the solubilization liquor for the determination of the total organic carbon (TOC) and the volatile fatty acid (VFA) concentrations. TOC concentration was determined by a TOC analyzer (Shimadzu, TOC-5000) equipped with a solid sample module (Shimadzu, SSM-5000A) and the VFA concentration by an organic acid analyzing system (Shimadzu, LC-10). The solubilization ratio was de®ned as the TOC concentration of the ®ltrate/the TOC concentration of the sludge before treatment 100. Similarly, the VFA formation ratio was de®ned as the TOC concentration of the VFA in the ®ltrate/the TOC concentration of the activated sludge 100.
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Methane fermentability measurement The granular sludge and UASB euent used for methane fermentation were obtained from the UASB reactor of the brewery's wastewater plant. The procedure used to measure methane fermentability was as follows: the sludge solubilized for 2 days was centrifuged at 2000 rpm for 15 min, after which the supernatant was diluted with the UASB euent to a COD value of 0.5 g/l. One l of this liquor then was added to 4 g of the granular sludge and the whole stirred at 358C under anaerobic conditions. The gas generated was bubbled through a 0.2 N solution of sodium hydroxide to eliminate carbon dioxide, after which the volume of methane gas was measured. Continuous treatment apparatus The experimental apparatus consisted of a semicontinuous, up¯ow column as the solubilizer, details of which are shown schematically in Fig. 1. It had a diameter of 257 mm, a liquid volume of approximately 50 l and was maintained at 60±638C with belt-heaters to promote selfdigestion. After the addition of sodium hydroxide at the ®nal concentration of 0.01 N and heating to 63±708C, sludge was supplied from the bottom of the column. A tube of the same diameter was attached at the top of the column, through which the solubilization liquor and the residual sludge was removed by over¯ow to a collection basin. This system was operated on an automatic 2 h cycle. For the ®rst 20 min of each cycle, the pump fed sludge into the column then stopped. After 100 min, it fed sludge again. The ¯ow rate was set at 200 ml/h, the hydraulic retention time being maintained for 24 h. The sludge used in this experiment was drawn from the same wastewater plant as the batch experiments and supplied for the apparatus directly.
RESULTS AND DISCUSSION
Conditions for self-digestion Figure 2 shows the time course of the solubilization ratio of excess sludge under several conditions. Solubilization proceeded rapidly for one day, after which the rate decreased. The solubilization ratio reached the maximum value in 4 days.
Fig. 1. Continuous treatment apparatus.
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Fig. 2. Time course of solubilization. Solubilized w: at 808C and 0.03 N NaOH; .: at 608C and 0.01 N NaOH; Q: at 608C without NaOH; R: at 378C without NaOH.
The eects of acid/alkali and the temperatures on solubilization were also examined. The results after 2 days of incubation are shown in Fig. 3. The solubility of the sludge increased with the rise in temperature and pH, whereas the VFA formation ratio was fairly high under neutral conditions. These ®ndings suggest that there were two types of solubilization; self-digestion accompanied by formation of these VFAs and alkaline solubilization which proceeds at high temperatures with poor VFA formation. When the liquor is to be treated anaerobically, the self-digestion method is
preferable to alkaline solubilization because the VFAs generated during self-digestion are the good substrates for methane fermentation. The less cost for solubilization is also favorable. In addition, a nearly colorless, clear solubilization liquor may be separated out by ¯otation without a conventional solid/liquid separation process. As far as the sludge was solubilized by selfdigestion, it seemed that the reactions proceeded insuciently at 508C, particularly without NaOH or HCl. A certain level of shock caused by changes of the surroundings might be required for selfdigestion. Above 708C with NaOH, the ratio of non-VFA was increased, indicating that the COD removal would be low in the anaerobic treatment. Thus, several conditions, such as a temperature of 608C and 0.01 N NaOH, temperatures of 70 or 808C without adding sodium hydroxide, were considered to be suitable for actual solubilization, evaluated on the basis of the solubilization ratio and VFA formation. The latter was advantageous in the point that the apparatus for alkaline addition is not required. The former, however, showed more stable results in spite of daily ¯uctuations of the sludge characteristics. Moreover, the temperature can be maintained easier in the case that the solubilization is performed with an open column. Therefore, we used the following conditions for the subsequent experiments: a temperature of 608C, 0.01 N NaOH (®nal concentration) and a retention time of 1 to 2 days.
Fig. 3. Eects of acid/alkali dose and temperature on solubilization. The additions and their ®nal concentrations were; (A) HCl 0.03 N; (B) HCl 0.02 N; (C) HCl 0.01 N; (D) none; (E) NaOH 0.01 N; (F) NaOH 0.02 N and (G) NaOH 0.03 N.
Solubilization of excess activated sludge
Fig. 4. Methane yield from the solubilization liquor. Solubilized .: at 608C with 0.01 N NaOH for 2 days; *: at 608C without NaOH for 2 days; R: at 808C with 0.03 N NaOH for 2 days; w: control with sodium acetate as the substrate; q: blank.
Methane fermentability Figure 4 shows the methane production from the solubilized matter. The rates were signi®cantly aected by the solubilization conditions. At 608C and 0.01 N NaOH, the rate was almost the same as in the control experiment with authentic acetate in place of the solubilization liquor. Judging from this, the methane fermentability of the liquor under this condition was excellent. At 608C without NaOH, the methane fermentation rate was 77% at 5 h as compared to authentic acetate, though there was no signi®cant dierence in the proportion of VFAs in the substrates. As the explanation for this reduction in the rate, the degradation of biopolymeric substances to lower-molecular-weight-compound might have been proceeded insuciently, or less likely, inhibitory substances to methane fermentation had been formed in the series of solubilization reactions. To clarify this, further investigations would be needed. On the other hand, when the sludge was solubilized at 808C and 0.03 N NaOH, methane production markedly decreased to 35% at 5 h as compared to authentic acetate. It was con®rmed that drastic heating and excess addition of NaOH caused the formation of substances that was not readily convertible to methane. These results indicate that the conditions of 608C and 0.01 N NaOH are suitable for the ecient anaerobic treatment followed by the solubilization.
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bilization by self-digestion, there is a possibility that no special gas generation process is required because enough gas was formed and the ¯otation occurred during self-digestion. If the ¯oating residue can be removed eectively in larger plants, a solid/liquid separation process would be unnecessary. The suspended solids (SS) value of the solubilization liquor was less than 15 mg/l in the batch experiments. Flotation of the sludge occurred within 1 h of heating. When ¯otation proceeded suciently, the speci®c gravity of the ¯oating sludge was reduced to 0.6±0.7 due to the gas bubbles existed in the ¯oating sludge. The thickening eciency, or the solid content of the ¯oating sludge paralleled the increasing depth of the sludge suspension in the containers (Fig. 5). The concentration and the volume of the sludge had no eect on the ®nal solid content of the ¯oating sludge cakes. Above a solid content of 10%, the sludge residue masses were sti and the solubilization liquor could be separated easily. Preliminary results indicate that the thickened residual sludge produced by this process had improved dewaterability than non-treated sludge. The ¯otation of solids was not always followed by self-digestion. The rise of the temperature, the lowering of the pH of the sludge suspension or both caused immediate ¯otation of the solids. This suggests that the release of carbon dioxide dissolved in the sludge liquor caused the ¯otation. Nevertheless, ¯otation occurred along with selfdigestion when the original carbon dioxide was removed by replacing the sludge liquor with distilled water. In this self-digestion process, it can be consequently said that both the original carbon dioxide and the gaseous product of self-digestion contributed to the ¯otation thickening and made the ¯oating sludge stable and sti.
Liquid/solid separation In a liquid/solid separation process prior to dewatering, ¯otation is used for the thickening of the sludge occasionally. The formation of gas bubbles which lift up the solids is said to be one of the critical steps for the eective ¯otation thickening. In the conventional ¯otation thickening techniques, physical, chemical and electrolytic methods are used for the gas generation. In case of solu-
Fig. 5. Correlation of initial depth of the sludge and solid content of the ¯oating sludge. The initial sludge concentrations and the volumes were Q: 0.71%, 100 ml; q: 0.90%, 100 ml; w: 0.90%, 500 ml.
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Fig. 6. Solids behavior in the solubilizer. (a) Solids fed into the solubilization column in each cycle were initially settled and thickened in the solubilizer. (b) As self-digestion proceeds, gas bubbles gradually form in the solids layers and lift up the solids. (c) The ¯otated solids undergo further solubilization then over¯ow from the top of the solubilizer.
Continuous treatment The behavior of the solids in the solubilization column was shown in Fig. 6. The proportion of the submerged sludge to the total ¯oating sludge was around 70%, indicating that the average speci®c gravity of the ¯oating sludge was around 0.7. When the ¯oating sludge was accumulated in the column, it was pushed and collapsed by the wall of the exhaustion tube and then ®nally exhausted from the top of the column along with the liquor. In the collection basin, the residue was immediately settled at the bottom of it because the gas bubbles adhering to the solids were released. In this experiments, the solubilization liquor could be separated from the solid±liquid mixture in the basin only by pumping. Operational results are shown in Fig. 7. The sludge concentration was varied between 0.7 and 1.4%, as it was drawn and supplied for the apparatus directly. Solubilization of approximately 20%
and VFA formation ratios of about 10% were obtained at a hydraulic retention time of 24 h. These ®ndings are very similar to those obtained in batch experiments under identical conditions. The solid content of the residue (the solids after selfdigestion) ranged from 15 to 17% at the top of the column. The SS values of the solubilization liquor ranged from 25 to 125 mg/l. On the basis of the fact that excess activated sludge is composed mainly of bacteria, the sludge could be solubilized by self-digestion. There are several merits to using self-digestion. One is that the conditions for self-digestion are milder than in other solubilization methods because self-digestion may occur when there is a change in the conditions as in pH and temperatures. When self-digestion occurred, the biopolymeric substances were hydrolyzed, forming VFAs that were easily converted to methane under anaerobic conditions. The usefulness of ¯otation of the solids during self-digestion also should be noted because it seems to be eective
Fig. 7. Daily solubilization and VFA formation ratios. w: solubilization ratio; .: VFA formation ratio. The solid and broken bars on the right show the range of the data obtained from the batch experiments under identical conditions.
Solubilization of excess activated sludge
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Fig. 8. Proposed wastewater treatment system.
for solid/liquid separation. Whereas most other research has regarded solubilization as pretreatment for anaerobic digestion, this approach may enable the combination of sludge solubilization and UASB treatment. A ¯ow diagram of the proposed process is shown in Fig. 8. This system combines sludge solubilization with the present wastewater treatment process that consists of aerobic treatment followed by anaerobic treatment using UASB. The excess activated sludge drawn from the sedimentation tank is solubilized and reduced by self-digestion. After solid/liquid separation, methane energy is recovered from the solubilized matter using a UASB reactor. With this system, excess sludge can be reduced economically, especially in the wastewater treatment plants that have anaerobic treatment systems. The time required for its treatment, including secondary UASB treatment, is markedly reduced; 2 to 5 days, depending on the solubilization ratio desired. Moreover, the low operation cost, ability to recover methane and the simplicity of the bench scale unit make it suitable for treating large quantities of sludge by self-digestion. Several factors, however, require clari®cation. In this study, the residual solids exhausted from the continuous apparatus were separated from the solubilization liquor by settling. If the ¯oating cake can be removed directly from the top of the solubilization column, the proposed process might be simpli®ed even further. Also, extensive dewaterability analyses have yet to be performed, the results of which may signi®cantly aect the cost of sludge disposal. Moreover, the odor problem should be considered when putting this system to practical use especially in the town area. The eectiveness of this process on sludges from other treatment plants also must be investigated. A pilot scale apparatus of the proposed self-digestion system has been constructed and is currently being tested. It is hoped that an ongoing approach will produce favorable ®ndings.
SUMMARY AND CONCLUSIONS
Our study of a self-digestion process to reduce excess sludge produced the following conclusions: 1. Solubilization (20±40%) of excess sludge by selfdigestion was possible. The selected self-digestion conditions were; a temperature of 608C, an anaerobic incubation time of 4 days and an NaOH concentration of 0.01 N. These conditions ensured approximately 40% solubilization of the organic matter in the excess sludge. 2. The gas bubbles formed during self-digestion caused thickening of the residual solids by ¯otation. 3. The subnatant liquor was eectively treated with anaerobic granular sludge, forming methane. 4. In the continuous treatment with a retention time of 24 h, the solubilization ratio was almost the same as that for the batch experiments done under identical conditions. AcknowledgementsÐThis research, one subject in the National Project for Development of Biosensors in the Food Industry, was supported in part by a grant from the Japanese Ministry of Agriculture, Forestry and Fisheries to Asahi Breweries, through the project implementation body, the Society for Techno-Innovation of Agriculture, Forestry and Fisheries.
REFERENCES
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Nagai S. and Nishio S. (1985) Rapid methane fermentation. J. Water Waste 27, 1134±1143 (in Japanese). Rajan R. V., Lin J.-G. and Ray B. T. (1989) Low-level chemical pretreatment for enhanced sludge solubilization. J. Water Pollut. Control Fed. 61, 1678. Ray B. T., Lin J. G. and Rajan R. V. (1990) Low-level alkaline solubilization for enhanced anaerobic digestion. JWPCF 62, 81±87. Shimizu T., Kudo K. and Nasu Y. (1992a) Ultrasonic pre-
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